Physics Letters A 318 (2003) 205–212 www.elsevier.com/locate/pla Design of the GG satellite A. Anselmi ∗ , G. Catastini Scientific Satellites Directorate, Alenia Spazio S.p.A., Turin, Italy Received 5 February 2003; accepted 28 July 2003 Communicated by V.M. Agranovich Abstract The design of the satellite for the GG experiment was addressed in phase-A level studies in 1997–2000, based on an equatorial orbit, and more recently re-addressed for sun-synchronous orbit (SSO). The mission consists of an experiment running uninterrupted with few operational modes, small telemetry rates, easily controlled by one ground station. The satellite is small, low-weight, with low power demand. The configuration, resembling a spinning top, is very compact and stiff. The main requirements are for thermal stability, drag-free control and spin rate control. The reconfiguration to SSO makes the mission suitable for a low-cost launch, and improves the thermal performance.  2003 Elsevier B.V. All rights reserved. PACS: 07.87.+v 1. Introduction GG is a small scientific satellite mission with the objective of testing the equivalence principle (EP) to 1 part in 10 17 , four orders of magnitude better than the best experiments to date. The experiment [1,2] con- sists in testing the universality of free fall by means of two concentric hollow cylinders of different composi- tion (the test masses) in orbit in the gravitational field of the Earth. Two additional cylinders, also concen- tric, made from the same materials, provide the zero- check. The cylinders spin around their common axis, perpendicular to the orbit plane, at a rate (2 Hz) much larger than the orbit rate. Weak suspensions constrain the motion to the plane perpendicular to the spin axis. * Corresponding author. E-mail address: aanselmi@to.alespazio.it (A. Anselmi). Suitable sensors (capacitance plates) located between the test masses detect the relative displacements. A vi- olation of EP would manifest itself as a displacement of the order of 1 pm, constant in the orbit frame and directed toward the center of the Earth. The test bodies, their mechanical coupling and the capacitance read-out are the core of the mission. Once the experimental design has been conceived, the required features of the spacecraft and its attitude and orbit follow. The spacecraft must provide a suitable accommodation to the experiment, provide specific and stable mass properties, shield it from thermal and dynamic perturbations within specified limits. The spin axis must be nearly perpendicular to the orbit plane, gyroscopic stiffness providing stability against external attitude disturbances. The orbit must be near circular, the altitude and inclination being selected as the best compromise between experiment needs and technical and cost considerations. 0375-9601/$ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2003.07.023